A Perspective

Michael E. Himmel, William S. Adney, John O. Baker, Richard Elander,
James D. McMillan, Rafael A. Nieves, John J. Sheehan,

Steven R. Thomas, Todd B. Vinzant, and Min Zhang

Biotechnology Center for Fuels and Chemicals, National Renewable
Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401

Conversion of the fermentable sugars residing in lignocellulosic waste and energy crops can conservatively yield approximately 100 billion gallons of fuel-grade ethanol per year in the United States alone. However, the cellulosic biomass-to-alcohol bioconversion process must be proven economical before industry can commercialize this technology. The U. S. Department of Energy, Office of Fuels Development supports a program to develop a commercially viable process for producing ethanol transportation fuel from renewable biomass resources. Bioconversion technologies developed to date take advantage of a diverse array of pretreatments, followed by enzymatic or chemical saccharification, and fermentation. Progress during recent years in pretreatment and fermentation technologies promises to significantly improve overall process economics. Examples include the development of two-stage dilute acid, nitric acid/mechanical disruption, and countercurrent percolation pretreatments. Recently developed, genetically engineered ethanologens also promise to improve process economics and include Escherichia coli, Saccharomyces sp., and Zymomonas mobilis. These microorganisms reportedly ferment xylose and glucose mixtures with high efficiency. This review presents an up-to-date picture of the advanced technology aspects of unit operations key to successful biomass-to-ethanol processing plants with speculation about future focus in this field by NREL researchers.